Analytical balances with which an extremely light object of approximately 1 μg can be measured have been proposed as electronic balances, and large scale balances with which an extremely heavy object of several tens of kg can be measured have also been proposed. In these electronic balances, a change in the temperature, for example, causes a change in the electromagnetic force, which is scaled with the load of the object to be weighed, or a slight expansion or contraction of the load conveying mechanism. As a result, there is an error in the results of the measurement of the load of the object to be weighed. Therefore, it is necessary to correct the electronic balance using weight for correction in order to prevent errors from being caused in the results of measurement of the load of the object to be weighed.
Thus, in the case where correction is carried out using weight for correction, it is necessary to pay attention to the handling and maintenance of the weight for correction, and therefore, there are weight incorporated type electronic balances where weight for correction has been installed in advance within an electronic balance. In these weight incorporated type electronic balances, appropriate correction is carried out through the operation of the buttons by the operator, or automatic correction is carried out through signals from a timer, a temperature sensor and the like.
FIG. 5 is a diagram schematically showing the configuration of a conventional weight incorporated type electronic balance. In an electronic balance 60, an upper beam 50, a lower bean 51 and a movable portion 52 for connecting the upper beam 50 and the lower beam 51 form a main Roberval mechanism R1 for conveying the load of the object to be weighed W which is applied to a scale 56 in the vertical direction, and a beam 54 which is connected to the movable portion 52 of the main Roberval mechanism R1 is provided so as to be able to sway with a fulcrum 55 at the center. Furthermore, an electromagnetic force generating apparatus 57 is provided to the beam 54 on the side opposite to the movable portion 52 side. As a result, the load of the object to be weighed W which is applied to the scale 56 is conveyed to the beam 54 via the main Roberval mechanism R1, and thus, an electromagnetic force required to keep the beam 54 balanced is generated in the electromagnetic force generating apparatus 57. Accordingly, the current value which is required to generate the electromagnetic force at this time is measured so that the load of the object to be weighed W can be measured as the amount of electricity. At this time, the ratio of the length (g) of the beam 54 on the movable portion 52 side to the length (f) of the beam 54 on the electromagnetic force generating apparatus 57 side with the fulcrum 55 of the beam 54 as a reference becomes the leverage.
In this electronic balance 60, correction is carried out using incorporated weight 58, and therefore, an engaging portion 62a is provided in the movable portion 52 and the load of the incorporated weight 58 can be conveyed to the engaging portion 62a in the configuration.
FIG. 6 shows an example of a conventional electronic balance where the size of incorporated weight is reduced, and in the electronic balance 160, correction is carried out using light incorporated weight 58, and therefore, an extension portion 54a which extends from the movable portion 52 side of the beam 54 is provided, and at the same time, the movable portion 61 of a sub-Roberval mechanism R2 is connected to the extension portion 54a. Here, the same symbols are attached to the same components as in the above described electronic balance 60, and the descriptions thereof are omitted.
An upper beam 63 and a lower beam 64 are connected to the top and the bottom of the movable portion 61, and thus, the sub-Roberval mechanism R2 is formed. In addition, an engaging portion 62b on which incorporated weight 58 is placed is provided in the movable portion 61. At this time, the ratio (h:f) of the length (h) of the beam 54 on the movable portion 61 side and the length (f) of the beam 54 on the electromagnetic force generating apparatus 57 side with the fulcrum 55 of the beam 54 as a reference becomes the leverage. Accordingly, the point where the movable portion 61 to which the load of the incorporated weight 58 is applied and the beam 54 are connected is moved away from the fulcrum 55 of the beam 54, that is to say, the length (h) of the beam 54 on the movable portion 61 side is increased, and thus, correction can be carried out with a relatively light incorporated weight 58.
Here, in order to carry out precise correction, it is desirable for the electromagnetic force to be generated in the electromagnetic force generating apparatus 57 so that the object to be weighed W is in a state which is actually measured using incorporated weight 58. However, in large scale balances with which an extremely heavy object to be weighed W of several tens of kg is measured, in the case where incorporated weight 58, which is sufficient for the object to be weighed W, is contained, it increases the weight of the electronic balance 60 or 160, while in the case where the length (h) of the beam 54 on the movable portion 61 side is increased in order to use the relatively light incorporated weight 58, various problems arise such that the volume of the electronic balance 160 increases.
Thus, incorporated weight type electronic balances having first and second beams are disclosed. FIG. 7 is a side diagram showing another conventional weight incorporated type electronic balance, and FIG. 8 is a diagram schematically showing the configuration of the electronic balance shown in FIG. 7. The electronic balance 80 is provided with a main Roberval mechanism R1 for conveying the load of the object to be weighed W which is applied to a scale 76 in the vertical direction, a first beam 74, which is formed so as to be able to sway by means of a fulcrum 75 and of which one end is connected to a movable portion of the main Roberval mechanism R1 and of which the other end is connected to a second beam 92 via a connection member 93, an electromagnetic force generating apparatus 77, incorporated weight 78 for correction, a sub-Roberval mechanism R2 for conveying the load of the incorporated weight 78 in the vertical direction and a second beam 92 of which one end is connected to a movable portion 81 of the sub-Roberval mechanism R2 and of which the other end is connected to the electromagnetic force generating apparatus 77. As a result, the ratio (j:i) of the length (j) of the first beam 74 on the movable portion side to the length (i) of the first beam 74 on the connection member 93 side with a fulcrum 75 of the first beam 74 as a reference becomes the leverage, and at the same time, the ratio (k:m) of the point (k) at which the connection member 93 and the second beam 92 are connected to the length (m) of the second beam 92 on the electromagnetic force generating apparatus 77 side with a fulcrum 91 of the second beam 92 as a reference becomes the leverage.
In this electronic balance 80 also, correction is carried out using incorporated weight 78, and therefore, an extension portion 92a which extends from the second beam 92 on the side opposite to the electromagnetic force generating apparatus 77 side is provided, and at the same time, a movable portion 81 of the sub-Roberval mechanism R2 is connected to the extension section 92a. Here, an upper beam 83 and a lower beam 84 are connected to the top and the bottom of the movable portion 81, and thus, a sub-Roberval mechanism R2 for conveying the load of the incorporated weight 78 in the vertical direction is formed. At this time, the ratio (l:m) of the length (l) of the second beam 92 on the movable portion 81 side to the length (m) of the second beam 92 on the electromagnetic force generating apparatus 77 side with a fulcrum 91 of the second beam 92 as a reference becomes the leverage. Accordingly, the point where the movable portion 81 to which the load of the incorporated weight 78 is applied and the second beam 92 are connected is moved away from the fulcrum 91 of the second beam 92, that is to say, the length (l) of the second beam 92 on the movable portion 81 side is increased so that correction can be carried out with a relatively light incorporated weight 78.